Manufacture of sulfuric acid



May 24, 1949. H. F. MERRIAM I MANUFACTURE OF SULFURIC ACID Filed April 3, 1946 Comm/5770M J01 FUR B ORA/7? OOH AP D/BQFT PRODUCT (ONVE/PI'E P JKSTEM BZOWEE INVENTOR E A/{RRMM BY W ATTORNEY Patented May 24, 1949 UNITED STATES ATENT OFFICE MANUFACTURE OF SULFURIC ACID Henry F. Merriam,

to Allied Chemical & D

poration of New York p Application April 3, 1946, Serial No. 659,4l0

or recovery system 'ajnd for convenience of discussion herein may be designated as a single phase S02 oxidation-S03 recovery system.

It has been proposed to make sulfuric acid by subjecting asuitable S02 oxygen gas mixture to the action of'an S02 oxidation catalyst under conditions to oxidize part but not all of the S02 to S03, separating the S03 from the gasstream by absorption in strong sulfuric acid, passing the gasstream containing oxygen and residual S02 through a second catalytic oxidation stage in which tlie remaining S02 is oxidized to S03, and then recoveringthe S03 thusformed in a second stron sulfuric acid absorber. For convenience of discussion herein, such an operationinclud ing'as a first phase, a first converter and a first absorber, and as asecondfphase a second converter and a second absorber-may be designated generally as a multiple phase S02 oxidation- S03 recovery systemjf While this type of process has been considered theoretically sound for many years, such oper'ationshave never become of commercial importance; In prior multiple S02 oxidatio'n S03 recovery systems, it has been proposed to catalytically oxidize a major portion, e. g.

60-80%, of the 'dry so; to so; in the first con.- version's'ta'ge, an then take the S03 resulting from first conversion out of. the gas stream by meansof the commonly known S03 absorption,

using for the latterpurpose strong sulfuric acid of the 'order of 98% strength. The attempts to utilize in the first phase of the operation, the combination of (1) catalytic oxidation of a major portion of'the dry S02 to S03, .(2) the known principles for absorption of 'SO out of a dry gas stream, and (3) strong sulfuric acid for S03 absorption have resulted the formation, in the gaseous eifiuent of the first absorber stage, of a peculiar sulfuric acid mist condition. Thismist has been of such arefractory nature that it has not been-possible to get rid'satisfactorily of such mist by the usual this refractory mist perists and causes corrosion of blowers, fines and heat exchangers.

methods of'filtration, e. g. by the use of coke filters, "etc, with theresult that West Orange, N. 1., assignor ye Corporation, a cor- Claims. (01. 2 3 1s7) paratory to utilizingsuchgases inthe-manufacture of sulfuric acid bythe contact process are well known, e. g. Herreshoff'U. S. Batents940,595' of November 16, 1909, and-1,1-13;437'ofOCtobr 1.3, 1914. A Wet'purification' system includesgat least one tower-in which the hot'in'comin'g S02 gas is contacted .countercurrent with a" stream of sulfuric acid; normal operation of thistower being such that the SQ-oxygen gaseQus-efiluent" contains sulfuricacid mist and water vapor. The predominating characteristics of-this type'of gas mixture .are a highwater vapor content most of which moisture may be condensed"anddischarged from the system as economically disposable weak sulfuric. acid, and-the presence of an acid mist of a type which -not 'onlymay be readily separatedfrom the gas streambutalso may be recoveredin usable form.

In accordance with the present invention, it has been found that the difiicultieswhich have madeit impracticable to use the above described multiple S02 oxidation-S03 recovery system in commercial operations may be overcome (a) by subjecting the. initial SOz-oxygen gas'mixture;

to the action of an S02 oxidation catalystunder conditions to oxidize a'substantial but prefer ably minor amount of the SO2 to S03, (b)t;by"

treating the resulting partiallyconverted S02 gas stream in accordance with the principles of wet purification as distinguished from the 'prin-' ciples of S03 absorption, and (c) by using in such' treating step sulfuric acid contacting medium of H2SO4 strength substantially'less than-usually employed in S03 absorption. By so doing, I have" found that instead of getting a diificultly 'man ageable acid mist at the end of the first phase'of operation, as is the case with theme in the first phase of the process of the knownSIJs absorp tion, I am enabled to obtain in the first phase of operation an acid mistsoamenable to handling that it may be readily removed'from the "gas" streamby usual methods of filtration'with the result that there. is no refractory acid mist'to';

pass into the second phase of operation. 1

Theobjects and advantages of the invention may be further understood from the following descriptiontaken in connection with the accom showing diagrammatically apthe improved process may be panying drawing paratus. in which carried out,

Referringto. the drawing," any suitable' sulfur" bearing material, such as brimstone, may b burned in burner ID with 'nnrlviqil 11; n..n 4.1.

along with air into combustion chamber 1 I. These two units may be operated as known in the art to form combustion chamber exit heated to temperature of 1400 to 1900" F. and containing by volume (dry basis) 8 to 12% S02, to oxygen, and varying quantities of water vapor. Such a gas stream is run thru a suitable cooler or radiator l2 functioning to cool the gas stream down to say 750-1100" F., but not lower than a temperature high enough to initiate catalytic oxidation of S02.

In accordance with one feature of the present improvements, the gas mixture is then subjected to the action of an S02 oxidation catalyst under conditions to oxidize a substantial but preferably minor amount of S02 to S03. The present process is particularly applicable to the handling of strong S02 gases, and while the gas stream entering the preliminary converter l3 may carry a deficiency of oxygen for commercial oxidation of total S02 to S03, such gas stream usually inherently contains more than sufficient oxygen to effect the S02 oxidation sought for in converter l 3. The latter may be of any suitable design and contain any satisfactory catalyst such as vanadium or even FezOa. Size of the catalyst particles, depth of the catalyst layer or layers and other factors known in this art may be so chosen that for any given set of operating conditions the catalyst body afiords no marked resistance to gas flow, and at the same time efiects oxidation of preferably less than 50% of the S02 to S03. Under the better conditions of operation, oxidation of S02 to S03 in converter 13 may be upwards of 25% but not more than 40 In this situation the amount of S02 oxidation effected in converter l3 usually is such as to leave in the gas stream enough S02 sothat the catalysis step of the second phase of the process under discussion may be operated on a self-sustaining basis, i. e. no extraneous heat is needed to initiate or maintain catalysis. Should conditions be such that it is desired to effect a high degree of S02 oxidation in converter l3, enough strong S02 gas may be bypassed thru line M to put the second phase catalysis on a self-sustaining basis.

In accordance with a second feature of the invention, the gas mixture exiting converter 13,

e. g. heated to temperatures of 000 to 1300 F.

and containing 6.5 to 9% S02, 2 to 5% S03, oxygen, nitrogen and moisture, is contacted with sulfuric acid of H2S04 strength not in excess of 93% under conditions to concentrate such acid and to cool the gas stream to temperature of not more than 330 F. by conversion of all or part of the sensible heat of gas to latent heat of vaporization of water thereby forming a contacting zone off-gas containing S02, acid mist, and a substantial amount of water vapor. This result is accomplished by introducing the gas stream from line [5 into the bottom of a gas cooling or sulfuric acid condensing tower 11.

Such tower, which may be packed as desired, is provided with a circulatin system designated as l9 by means of which the H2S04 strength and temperature of the acid fed into the top of the tower may be controlled and by which the rate of circulation of such acid over the tower may be regulated. The countercurrent contacting operation carried out in tower I1 is characterized principally (1) by an approximate 0.5-1.5% increase of the I-I2SO4 concentration of the sulfuric acid liquor during one pass of acid over the tower, (2) by cooling of the gas stream to not more than 330 F., preferably in the range of ZOO-300 F.,

such cooling, aside from minor radiation loss, being eifected substantially by conversion of sensible heat of the gas stream to latent heat of vaporization of water, and (3) by the introduction into the gas stream of a relatively large amount of water vapor. In accordance with another feature of the invention, it has been found that circulating system l9 should be operated so that the acid effluent of the cooling tower in line 22 has an H2804 strength not in excess of about 93%, i. e. so that the gas passing upwardly thru the tower contacts a downfiow of acid of not more than 93% strength. Such acid strength may vary from 60 to not more than 93%, but is preferably of strength varying from 60 to not more than Temperature of the acid effluent in line 22 may vary from F. to 450 F. depending chiefly on the strength of the acid product being made by tower l1. Thus, for a given gas flow into the bottom of tower ll, temperature, strength and quantity of the acid fed into the top of the tower are controlled so that the acid effiuent leaving the bottom of the tower is at least ightly more concentrated than the acid input, is within the temperature range of about 160 to 450 F., and so that the temperature of the gas stream leaving the top of the tower is preferably not more than 300 F. and usually in the range of ZOO-300 F. It has been found that under the foregoing conditions some of the S03 entering the bottom of tower ll unites with water and is condensed to H2804, that the gas stream leaving the top of tower ll contains as I-I2S04 mist the balance of the S03 entering the bottom of tower ll, and that this mist is readily amenable to separation from the gas stream. Further, such gas stream contains a substantial excess of water vapor over that needed to combine with all S03 present therein to form H2S04.

Next, the gas stream containing sulfuric acid mist and excess moisture is cooled to temperature low enough to condense water vapor and facilitate removal of the acid mist. For this purpose, the gas stream may be run from line 2| into the top of a cooler 2 over the outer surface of which cold water may be flowed. Control of this cooler is such as to cool the gas stream to such an extent that the gaseous eiiluent leaving thru line 25 is at temperature of about '75110 F. Under these conditions most of the water condenses out and is discharged from the cooler thru line 26 as a liquor containin say e-6% H2SO4, i. e. weak enough to permit economic disposal to waste if necessary. The gas stream containing residual S02, acid mist, some oxygen and a minor amount of moisture is run into the filter 28 which may contain a bed of coke of such varying area and depth as to effect precipitation and filtration of all of the acid mist contained in the gas stream. This acid mist runs out of the bottom of the filter thru line 29 as aqueous sulfuric acid of strength which may range from 10 to 30% H2SO4. Such liquor is run thru line 29 into circulating system is to return thereto the H280; collected by filter 28, which H2S04 is eventually recovered as product sulfuric acid at liquor off-take 5| of tower I1. Thus it will be seen that in the first phase of the process of this invention, 1. e. the procedure up to the as outlet of filter 28, the SO: formed in converter I3 is recovered as product sulfuric acid in a gas-liquid sulfuric acid contacting operation. the oiT-gas stream (in conduit 2|) of which contains S02, sulfuric acid mist, and a substantial amount of water vapor. This mist is of a type which is readily removed from the gas stream by simple coke filtration, with the result that no 're fractory or persistent mist is contained in the S02 gas stream as the latter enters the second" phase of the process. While the chemical and/ or physical phenomena involved are not fully known, it is believed that the presence of a substantial amount of water vapor in the off-gas of tower i is the factor to which readily separable nature of the contained acid mist maybe attributed. It appears that, possibly'because of absorption of water, the acid mist particles become increased to a size which permits'thorough removal by simple filtration means.

'The gaseous substantially mist free ellluent of the filter in line '31 contains S02, oxygen, nitrogen and a small amount of moisture. Ordinarily, especially when utilizing in the first instance strong S02 gas, the gas in line Si is deficient in oxygen to support catalytic oxidation of residual S02 to S03. In this situation whatever additional air may be necessary to supply the needed oxygen may be introduced at inlet .33. Such gas stream is then run thru line 35 and into the bottom of drying tower 3'! provided with a. drying acid circulating system 39.. After drying, a typical gas stream in line Mmayconta'in 6.5 to S02, and 6.5 to 10% oxygen, the balance being nitrogen, possibly some 002, etc. By means of blower 44 the gas stream is passed into and thru the converter system 55 and the resultant S02 is recovered as product sulfuric acid by absorber 46, operation of the process from drying tower 31 thru to the recovery of 98% H2804 from absorber 45 being substantially the same as in known practice. Converter system 45 may contain one or more converters and heat exchangers, e. g. as shown in Fowler U. S, Patent 1,930,125 of October 10, 1933. The tail gas leaving absorber 46 thru exit 48 may contain a small amount of acid mist unavoidably formed in the absorption step.

The process of the invention is particularly adaptable for handling wet, strong S02 gas. Since any excess water over and above that needed may be eliminated from the process thru downdraft cooler outlet 26, it will be seen that, provided a water insensitive catalyst is used in preliminary converter l3, no drying of the gas stream up to and including the coke filter 28 is necessary. In addition to the brimstone gas described, strong S02 gas obtained by combustion of I-I2S may be employed. Also, strong S02 gas resulting from the thermal decomposition of acid sludge may be introduced into combustion chamber I l in which temperatures may be raised high enough to oxidize organic impurities to carbon dioxide, and the resulting gas passed thru the process as described.

In practice of the process of the invention, it is desirable to use an initial S02 gas containing not less than 10% S02 by volume, and preferably a gas containing 12% or more S02. The initial gas stream may be high in S02, e. g. 12-14%, and substantially deficient in oxygen for catalytic oxidation of the total S02 to S03. By means of the partial conversion step described it is possible to take part of the acid product out of the gas stream in the first phase of the process without the use of the substantially larger apparatus which would be necessary if it were required to carry in the gas stream in the forepart of the process all the oxygen needed for S02 oxidation. In the latter phase of the process, i. e. from the outlet of coke filter 28 on, S02 strength is usually appreciably less than 10%, and accordingly the apparatus in the last part of the process need not be designed to'handle a wrong-S02 gas. existing contact sulfuric acid plants are designed to handle S02 gas-es not stronger than about-140%. Byemploy ing the partial conversion stage 13 of the instant process, an initial S02 gas containing 12% S02 and 9% 02" would contain atthe head of converter system about 7.2%SO2 and 8.1% 02, and-hence no appreciable change'indesign of the plant "is necessary. In the case of aiplant designed to handle an S02 gas of 10%maximum strength, it will be 'seen'thait by starting with a 12% S02 gas overall plant production may be increased 20%.

Sulfuric acid may be recoveredsi-n several marhetable "strengths--the 98% I-I2SO4 recovered from absorber 46, and as the product which may be recovered as make from circulating system [9 0f coolingtower IV. In accordance-with-particularly desired operating c0nditi0ns,"the drawoif from circulatingsystem 19 at 51' may bebf strength-from l-llto 93%, it being particularly,

feasible to operate tower 11 so' that'the products thereof may be Be. -acid, 77.l% TH2'S04 strength, or 66% B. :acid, 93. 2% H2804 strength.

i I claim:

1. In the manufacture of sulfuric aci-d'by the contact "process, the steps comprising iorming an S02-oxygen gas mixture heated to temperature high enough to initiate catalytic oxidation, subjecting the gas mixture to the action of an S02 oxidation catalyst under conditions to catalytically oxidize a substantial but minor amount of said S02 to S02, contacting the resulting gas stream in a gas-liquid contacting zone with sulfuric acid of I-I2S04 strength not in excess of 93%, and in said zone, substantially by converversion of sensible heat of the gas to latent heat of vaporization of water, boiling water out of said acid and into the gas stream as water vapor, concentrating said acid, and cooling the gas stream to temperature not more than 300 F., whereby there is formed a contacting zone exit gas stream containing S02, acid mist and water vapor, cooling the gas stream containing acid mist and moisture to temperature low enough to condense the bulk of the water vapor as weak sulfuric acid and to facilitate removal of acid mist from the gas stream, separating acid mist from the gas stream, drying the gas stream, subjecting residual S02 to the action of an S02 oxidation catalyst, and recovering resulting S02 as sulfuric acid.

2. In the manufacture of sulfuric acid by the contact process, the steps comprising forming an S02-oxygen gas mixture heated to temperature high enough to initiate catalytic oxidation, subjecting the gas mixture to the action of an S02 oxidation catalyst under conditions to catalytically oxidize a substantial but minor amount of said S02 to $03, contacting the resulting gas stream in a gas-liquid contacting zone with sulfuric acid of H2S04 strength not in excess of 93%, and in said zone, substantially by conversion of sensible heat of the gas to latent heat of vaporization of water, boiling water out of said acid and into the gas stream as water vapor, concentrating said acid, and cooling the gas stream to temperature not more than 330 F., whereby there is formed a contacting zone exit gas stream containing S02, acid mist and water vapor, and a contacting zone liquid effluent comprising intermediate strength product sulfuric acid, cooling the gas stream containing acid mist and moisture to temperature low enough to condense the bulk of the water vapor as weak sulfuric acid and tofacilitate removal of acid mist from the gas stream, separating acid mist from the gas stream as liquid of HzSOt strength substantially greater than that of said weak sulfuric acid, returning liquefied mist to said contacting zone, drying the gas stream, subjecting residual S02 to the action of an S02 oxidation catalyst, and recovering resulting S03, as high strength product sulfuric acid, by absorption in high strength sulfuric acid.

3. In the manufacture of sulfuric acid by the contact process, the steps comprising forming an SO2-oxygen gas mixture heated to temperature high enough to initiate catalytic oxidation, subjecting the gas mixture to the action of an S02 oxidation catalyst under conditions to catalytically oxidize a substantial amount but not more than 40% of said S02 to S03, contacting the resulting gas stream in a gas-liquid contacting zone with sulfuric acid of H2804: strength not in excess of 80%, and in said zone, substantially by conversion of sensible heat of the gas to latent heat of vaporization of water, boiling water out of said acid and into the gas stream as Water vapor, concentrating said acid, and cooling the gas stream to temperature in the range of 200-330 F., whereby there is formed a contacting zone exit gas stream containing S02, acid mist and water vapor, and a contacting zone liquid eflluent comprising intermediate strength product sulfuric acid, rapidly cooling the gas stream containing acid mist and moisture to temperature of '75-110 F. to condense the bulk of the Water vapor as weak sulfuric acid and to facilitate removal of acid mist from the gas stream, separating acid mist from the gas stream as liquid of H280; strength substantially greater than that of said weak sulfuric acid, returning liquefied mist to said contacting zone, drying the gas stream, subjecting residual S02 to the action of an S02 oxidation catalyst, and recovering resulting S03, as high strength product sulfuric acid, by absorption in high strength sulfuric acid.

HENRY F. MERRIAM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,789,460 Clark Jan. 20, 1931 2,071,598 Girsewald et al. Feb. 23, 1937 

